Data communication typically occurs as the transfer of information from one communication device to another. This is typically accomplished by the use of a modem located at each communication endpoint. In the past, the term modem denoted a piece of communication apparatus that performed a modulation and demodulation function, hence the term “modem”. Today, the term modem is typically used to denote any piece of communication apparatus that enables the transfer of data and voice information from one location to another. For example, modem communication systems use many different technologies to perform the transfer of information from one location to another. Digital subscriber line (DSL) technology is one vehicle for such transfer of information. DSL technology uses the widely available subscriber loop, the copper wire pair that extends from a telephone company central office to a residential location, over which communication services, including the exchange of voice and data, may be provisioned. DSL devices can be referred to as modems, or, more accurately, transceivers, which connect the telephone company central office (CO) to the user, or remote location, typically referred to as the customer premises (CP). DSL communication devices use different formats and different types of modulation schemes and achieve widely varying communication rates. However, even the slowest DSL communications devices achieve data rates far in excess of conventional point-to-point. modems.
Some of the available modulation schemes include pulse amplitude modulation (PAM), quadrature-amplitude modulation (QAM), carrierless amplitude/phase (CAP) and discrete multi-tone (DMT). Trellis coding is a well-known technique for encoding the modulated signal in order to reduce errors. In conventional trellis coding, a redundant bit is generated for some number of transmitted symbols and the redundant trellis bit is added to adjacent symbols. Unfortunately, in some communication systems, such as those that use PAM encoding with 8-bit (256 level) mu-law analog-to digital converters, which operate at a sample rate equal to the symbol rate of the transmitter, the quantization noise (the noise that results when low capacity analog-to-digital converters are used) can be so severe that local receiver equalization is impractical. In such a case, the remote transmitter pre-equalizes the transmit signal such that the receive analog-to-digital converter slices the signal at its 256 prescribed levels. In dial modems, which use the switched telephone network, one of the dominant impairments is echo from the local mu(μ)-law transmitter. The mu-law transmitter generates echo quantization noise, which varies from symbol to symbol depending on the mu-law segment being transmitted. Conventional communications devices use a precoder to whiten the noise. This is appropriate for channels that have linear distortion and white noise. Unfortunately, the quantization noise on the echo is independent of the channel and varies from symbol to symbol. The remote transmitter has no knowledge of the signal transmitted by the local transmitter and the remote precoder and pre-equalization filter cannot compensate for the quantization noise at the local transmitter.
This quantization noise typically spans adjacent symbols, thereby making convention trellis coding, which encodes adjacent symbols, ineffective because adjacent symbols may be corrupted with the noise. DSL modems, such as symmetric DSL (SDSL), utilize conventional Tomlinson precoders, which require the use of square signal constellations without shaping gain. Interleaving, such as that defined in commonly assigned U.S. Pat. No. 4,677,625, to Betts et al., entitled “Distributed Trellis Encoder,” can protect the trellis encoders in these DSL modems.
Therefore, it would be desirable to trellis code a transmit signal such that the effect of quantization noise on adjacent transmit symbols can be minimized or eliminated.